Overhauling code callout handling

* Fixed previous code callout handling that was outputting all code blocks with callouts as Docbook passthroughs.  Now they only output as Docbook passthroughs if there are other inline elements (e.g., emphasis), or if the corresponding calloutlist element is located in a distant location.

* Updated calloutlist handling to output as a Docbook passthrough if the corresponding code block was output as a Docbook passthrough.